What a key is, and what it tells you

A key is a shortlist

Of the twelve notes available, a song in a given key spends almost all of its time on seven of them. Those seven notes build seven chords, and those chords are the ones the song will use. That is the whole idea: a key is a shortlist of chords that sound like they belong together, and a song is mostly a walk around that list.

Take G major. Its chords are G, Am, Bm, C, D and Em. Play any of them after any other and it sounds intentional. Play a B♭ in the middle and every listener hears something happen — not necessarily wrong, but definitely an event. That is the difference between a chord in the key and one from outside it, and it is why knowing the key tells you what to expect before you have heard the song.

• G
• C
• D
• Em

Major and minor

The same seven notes can be organised around two different homes. Settle on the first of them and you get a major key, which is the bright one. Settle on the sixth instead and you get its relative minor, which is the sombre one — same notes, same chords, different centre of gravity.

G major and E minor are the same shortlist. What separates them is which chord the song keeps coming back to and which chord it finally stops on. This is also why an automatic estimate can honestly land on either: the chords alone do not always settle it, and a song that ends on Em is a stronger argument than any amount of counting.

How this site works it out

The estimate is arithmetic, not opinion. Each of the twenty-four possible keys is scored on how much of the song, measured in seconds rather than in changes, is spent on chords that belong to it. A chord that sounds for a minute counts for more than one that passes in a bar. The tonic gets a small extra weight, because a song that keeps returning home is making a claim about where home is.

If the best-scoring key does not account for at least three quarters of the song, the page says nothing about the key at all. That threshold is there on purpose. A song that modulates halfway through, or borrows heavily from a neighbouring key, has no single honest answer, and inventing one would be worse than leaving the line out.

Four chords, over and over

Number the chords of a major key I, ii, iii, IV, V and vi. An enormous share of popular music is built from four of them: I, V, vi and IV. In G major that is G, D, Em, C. In C major it is C, G, Am, F. In D it is D, A, Bm, G.

This is why learning a handful of shapes gets you so far so quickly, and why so many songs feel familiar the first time you play them. It is also the practical reason to care about the key: once you recognise the pattern, you are not learning four unrelated chords, you are learning one pattern in a new position.

Moving a song to suit you

Knowing the key is what lets you change it. If a song sits too high for your voice, shifting every chord down by the same number of semitones moves the whole thing without disturbing a single relationship inside it. Down two semitones, G C D becomes F B♭ C, and the song is identical apart from being lower.

On a guitar there is an easier version of that move in one direction: a capo. Clamp it on and every shape you already know now sounds higher, which turns the awkward keys into the comfortable ones without your left hand learning anything new. That is what the capo suggestion on a song page is doing — see barre chords and the capo.

When to distrust the line

The key on a song page is derived from chords that were themselves estimated by a machine listening to a recording. Two layers of estimate, and each can slip. If a song borrows a chord for effect, the detector may hear it as belonging, and the key drifts a step. If the recording is dense or heavily distorted, the chords underneath the estimate may be wrong to begin with.

The check takes ten seconds: play the chord the page names as the key and see whether the song sounds like it wants to end there. If it does, the line is right. If it sounds unfinished, try the relative minor — three semitones down from a major, and by far the most common way for the estimate to be off.